1. Field of the Invention
This invention relates to structures and methods of assembly of integrated circuit chips. More particularly, this invention relates to post-passivation technologies with metal caps.
2. Description of the Related Art
Copper interconnection requires an aluminum cap at the passivation openings to protect the copper from environmental deterioration such as oxidation from the ambient and to provide a metal pad for wire bonding. Today many integrated circuit chips use copper as the interconnection metal. From a performance perspective, copper interconnection offers a higher propagation speed than does an aluminum interconnection, making copper a desirable technological solution for current IC design. However, copper interconnection also incurs reliability concerns. When a copper I/O pad is exposed to atmosphere, its surface is subjected to chemical attack by the oxygen and moisture in the atmosphere. To overcome this problem, prior art has disclosed a method and structure to prevent copper chemical attack. By depositing a metal (such as aluminum (Al)) cap layer on the surface of the copper I/O pad, the copper I/O pad can remain intact in the passivation opening in the ambient. This metal cap layer is especially important where processing through the passivation layer is performed in one fab and then post-passivation processing is performed in another fab. Moreover, an Al (or other metal) pad is able to form a stable bonding structure with Au wire. Copper alone cannot form a bondable structure with Au wire. Therefore, the Al cap layer provides the wire-bonding capability for the copper I/O pad. FIG. 1 shows an aluminum cap 32 on a copper line 24. The Al cap allows the formation of a wire bond 40 attaching to it firmly. For example, U.S. Pat. No. 6,451,681 to Greer and U.S. Pat. No. 6,376,353 to Zhou teach using an Al cap over a copper bond pad for wire bonding. U.S. Pat. No. 6,544,880 to Akram discloses gold over a copper pad and optionally additional metals to prevent formation of intermetallic compounds in wire bonding.
U.S. Pat. Nos. 6,495,442 and 6,383,916 to M. S. Lin et al disclose a post-passivation interconnection process. The continued emphasis in the semiconductor technology is to create improved performance semiconductor devices at competitive prices. This emphasis over the years has resulted in extreme miniaturization of semiconductor devices, made possible by continued advances of semiconductor processes and materials in combination with new and sophisticated device designs. Most of the semiconductor devices that are at this time being created are aimed at processing digital data. There are however also numerous semiconductor designs that are aimed at incorporating analog functions into devices that simultaneously process digital and analog data, or devices that can be used for the processing of only analog data. One of the major challenges in the creation of analog processing circuitry (using digital processing procedures and equipment) is that a number of the components that are used for analog circuitry are large in size and are therefore not readily integrated into devices that typically have feature sizes that approach the sub-micron range. The main components that offer a challenge in this respect are capacitors and inductors, since both these components are, for typical analog processing circuits, of considerable size.
One of the problems that is encountered when creating an inductor on the surface of a semiconductor substrate is that the self-resonance that is caused by the parasitic capacitance between the (spiral) inductor and the underlying substrate will limit the use of the inductor at high frequencies. As part of the design of such an inductor it is therefore of importance to reduce the capacitive coupling between the created inductor and the underlying substrate. Co-pending U.S. patent applications Ser. Nos. 10/445,558, 10/445,559, and 10/445,560 apply the post-passivation process of U.S. Pat. No. 6,383,916 in addition to creating high quality electrical components, such as an inductor, a capacitor or a resistor, on a layer of passivation or on the surface of a thick layer of dielectric.